1. Field of the Invention
The present invention relates to a thin-film solar cell module, more particularly, a thin-film solar cell module including a solar cell element having a semiconductor layer of amorphous silicon.
2. Description of Related Art
Thin-film solar cell modules require a smaller amount of semiconductor material to be used than solar cell modules using crystal wafers. Also they can be formed on low-priced substrates such as glass substrates, metal substrates and the like by a low-temperature process. Therefore, the thin-film solar cell modules are expected to drop in price.
Among the thin-film solar cell modules, amorphous silicon solar cell modules are under active development since they are advantageous in that silicon, their material, exists in abundance and in that their material is harmless and does not affect the environment compared with compound solar cell modules using Cd and Se.
Among the amorphous silicon solar cell modules, thin-film solar cell modules of a light transmission type are also being developed which have the light take-in function of transmitting a part of incident light to the rear side in addition to the power generating function.
Generally, the amorphous silicon solar cell modules are roughly divided into the following two types according to their structure:
In one type, a transparent electroconductive film of SnO2, ITO, ZnO or the like is formed on a light-transmissive insulative substrate of glass or the like. A p layer, an i layer and an n layer of amorphous semiconductors are sequentially laminated on the transparent electroconductive film to form a photovoltaic conversion layer, on which a transparent electroconductive film of ITO, ZnO or the like and a metal film of Ag, Al or the like are sequentially laminated to form a rear electrode.
In the other type, a transparent electroconductive film of ITO, ZnO or the like is formed on a metal substrate. A p layer, an i layer and an n layer of amorphous silicon semiconductors are laminated on the transparent electroconductive film to form a photovoltaic conversion layer, on which a transparent electroconductive film of SnO2, ITO, ZnO or the like is formed.
Among these modules, in the amorphous silicon solar cell modules using light-transmissive insulating substrates of glass or the like, the following two processes are typically used for forming a light-transmissive portion for transmitting a part of incident light:
One is a wet process in which the rear electrode is formed partially on the photovoltaic conversion layer and the photovoltaic conversion layer exposed out from the rear electrode is etched away using the rear electrode as a mask.
The other is a dry process in which the photovoltaic conversion layer and the rear electrode are partially removed simultaneously by laser scribing.
After the light-transmissive portion is thus formed by either one of the processes, a rear electrode side is sealed by bonding a glass plate with an adhesive such as ethylene-vinyl acetate (referred to as EVA hereinafter in this specification), polyvinyl butyral (referred to as PVB hereinafter in this specification) or the like.
Materials for sealing the rear electrode side include a filler of EVA or PVB, a transparent PET sheet, a PET/aluminum film/PET laminate sheet, a transparent back sheet of Tedlar(copyright)(DUPONT) in addition to the glass plate.
The thus produced thin-film solar cell module of the light transmission type is placed outdoors for use with a frame of aluminum, stainless steel or the like attached to its periphery.
Typically the thin-film solar cell module of the light transmission type (referred to simply as xe2x80x9ctransmission type modulexe2x80x9d hereinafter in this specification) has a light-transmissive portion occupying about 1 to 50% of a power generating area, and therefore, has a lower power generation efficiency per unit area than a thin-film solar cell module of a non-transmission type (referred to simply as xe2x80x9cnon-transmission type modulexe2x80x9d hereinafter in this specification).
Accordingly, for generating a certain amount of power, more transmission type modules are required to be installed and occupy a larger installation area than the non-transmission type modules.
Under the above-described circumstances, it is desired that the transmission type modules have particularly high power generation efficiency. The modules in this specification mean a plurality of solar cells formed on a substrate which are electrically connected to each other.
On the other hand, typical amorphous silicon solar cell modules suffer an early-stage deterioration of conversion efficiency called Staebler-Wronski effect (referred to as xe2x80x9cphoto degradationxe2x80x9d hereinafter in this specification).
The photo degradation is a problem where an amorphous silicon semiconductor is used for the photovoltaic conversion layer of a solar cell module which generates power from solar light outdoors.
There has not been found a method for completely eliminating the photo degradation of the amorphous silicon solar cell modules. However, a method is known for reducing the photo degradation by thinning the thickness of the amorphous silicon semiconductor layer in a stacked solar cell module of tandem or triple structure in which unit cells are stacked in two layers or three layers.
It is known that the conversion efficiency dropped by the photo degradation is recovered by raising the temperature of the amorphous silicon solar cell modules (referred to as xe2x80x9canneal effectxe2x80x9d). This anneal effect can be observed at temperatures of about 40xc2x0 C., but the effect is enhanced at higher temperatures. For example, the conversion efficiency is known to be greatly recovered at a temperature of about 70xc2x0 C. or higher.
The temperature dependence of the output of the amorphous silicon solar cell modules is considerably smaller than that of the crystalline silicon solar cell modules. The output of the amorphous solar cell modules decreases about 0.1 to 0.2% when temperature rises about 1xc2x0 C. Therefore, if the amorphous solar cell modules are used with keeping a high temperature, the conversion efficiency improves more owing to recovery from the photo degradation than the output drops due to the increased temperature.
In other words, if the amorphous silicon solar cell modules are used with keeping a high temperature to recover the photo degradation, the photo degradation is reduced and a high output can be obtained.
As a particular method for taking advantage of this temperature characteristic of the amorphous silicon solar cell modules to reduce the photo degradation, there is generally known a method of suppressing the radiation of heat of solar light from the rear face of the amorphous silicon solar cell modules by providing a thermal insulator to the rear face (see Japanese Unexamined Patent Publication No. HEI 4(1992)-71276, for example).
However, this prior-art technique has the following problems.
The provision of the thermal insulator to the rear face of the amorphous silicon solar cell modules (referred to simply as xe2x80x9csolar cell modulesxe2x80x9d hereinafter in the specification) raises the highest temperature of the solar cell modules to about 70xc2x0 C. in the daytime of summer. However, the temperature of the solar cell modules does not exceed about 70xc2x0 C. in the other seasons.
Further, it is known that, if the above-described solar cell module having the thermal insulator is provided with a frame in its periphery for enhancing its strength, thermal conduction from the solar cell module to the frame increases.
That is, the heat of the solar cell module, especially of its periphery, is conducted to the frame and then radiated from the frame into the air. For this reason, the temperature of the periphery of the solar cell module, which is near the frame, is often lower by about 20xc2x0 C. than the temperature of the center of the solar cell module.
Japanese Unexamined Patent Publication No. HEI 11(1999)-103086 discloses a method of suppressing the thermal conduction to the frame by providing a thermal insulator between the solar cell module and the frame.
However, this prior-art technique has the following problem.
(1) The insertion of the thermal insulation between the solar cell module and the frame decreases installation strength of the solar cell module and mechanical strength of the solar cell module.
(2) The thermal insulator is often made of polystyrene, which may cause environmental pollution, or polyurethane, polyethylene or the like which involves the risk of generation of dioxin during usual incineration. These are not suitable for the solar cell module, which is installed in a large area, for example, on a roof. On the other hand, if PET and phenol resin, which are little likely to cause environmental pollution, are used for thermal insulation, these materials need to be formed in a thickness of 20 cm or more for keeping the temperature of the module having a large area since thermal insulation coefficients of the materials are about 0.3. Therefore, disadvantageously, the size of the module itself increases and a large quantity of the thermal insulator is disposed of at the disposal of the module.
(3) The thermal insulator is usually bonded to the rear face of the module with an adhesive, which requires an additional man-hour after the module is produced. Moreover, the adhesion of the thermal insulator to the module is often insufficient, and therefore, the adhesive is deteriorated by infiltration of water or the like during long-term outdoor exposure and the function of the solar cell module declines. In order to suppress this deterioration, an additional layer of a waterproof sheet must be provided, which further increases production costs.
(4) The periphery of the solar cell module needs to be sealed to prevent the infiltration of water into the semiconductor layer. If the thermal insulator inserted between the solar cell module and the frame has an insufficient weatherability, the sealing property of the thermal insulator is liable to decline greatly owing to deterioration of the thermal insulator by light, and therefore the output of the solar cell decreases. Especially, where an expanded resin which has an excellent thermal insulation property is used as the thermal insulator, the installation strength and sealing property decline greatly.
The above-described prior techniques of providing the thermal insulator on the rear side of the solar cell module and between the solar cell module and the frame are not intended for the transmission type module. Accordingly, naturally, opaque thermal insulators have been used usually.
The present invention has been made under the above-described circumstances and an object thereof is to provide a thin-film solar cell module capable of suppressing the photo degradation and providing large output.
Certain embodiments of the present invention provide a thin-film solar cell module of a light transmission type comprising a light-transmissive substrate; a front electrode layer; a photovoltaic conversion layer; and a rear electrode layer. The front electrode layer, the photovoltaic conversion layer and the rear electrode layer are sequentially laminated on the light-transmissive substrate. A heat retention member covers the rear electrode layer, and a sealing layer is provided for sealing the rear electrode layer, wherein the heat retention member has a light absorptance of 40% or more within a near-infrared wavelength range of 1,500 to 2,000 nm.
The heat retention member may be formed of glass containing 1 to 50% by volume of gas therein.
The heat retention member may have a sealing layer and a thermal insulation layer formed on the sealing layer and the thermal insulation layer may be formed of a sheet-form silica aerogel.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.